ARM: dts: at91: sama5d3_xplained: not all ADC channels are available

Remove ADC channels that are not available by default on the sama5d3_xplained
board (resistor not populated) in order to not create confusion.

Signed-off-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Cc: <stable@vger.kernel.org> # 3.16+
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>